An ELISA-based method to quantify osteocalcin carboxylation in mice

Osteocalcin: A bone protein with multiple endocrine functions

Bones are now recognised as endocrine organs with diverse functions. Osteocalcin, a protein primarily produced by osteoblasts, has garnered significant attention. Research into osteocalcin has revealed its impact on glucose metabolism and its unexpected endocrine role, particularly in its undercarboxylated form (ucOC). This form influences organs, affecting insulin sensitivity and even showing correlations with conditions like type 2 diabetes and cardiovascular diseases. However, analytical challenges are impeding advances in clinical research. Various immunoassays like RIA, EIA, ECLIA, IRMA, and ELISA have been developed to analyse osteocalcin. Recent innovations include techniques like OS-ELISA and OS phage Immuno-PCR, enabling fragment analysis. Advancements also encompass porous silicon for detection and ECLIA for rapid measurements. The limitations of immunoassays lead to ucOC measurement discrepancies, prompting the development of mass spectrometry-based techniques. Mass spectrometry increasingly quantifies carboxylated, undercarboxylated, and fragmented forms of osteocalcin. Mass spectrometry improves routine and clinical analysis accuracy. With heightened specificity, it identifies carboxylation status and serum fragmentations, boosting measurement reliability as a reference method. This approach augments analytical precision, advancing disease understanding, enabling personalised medicine, and ultimately benefiting clinical outcomes. In this review, the different techniques for the analysis of osteocalcin will be explored and compared, and their clinical implications will be discussed.

Metabolic shifts in ratio of ucOcn to cOcn toward bone resorption contribute to age-dependent bone loss in male mice

The study of the senile osteoporosis in men still lags significantly behind that in women. The changes of protein molecule levels and their relationships with bone loss remain poorly understood. In the present study, we used C57BL/6J male mice at ages from 3 to 24 mo to delineate the mechanisms of aging effects on bone loss. We used the microcomputed tomography, mechanical testing, histomorphometry assays, and detection of serum levels of undercarboxylated osteocalcin (ucOcn) and carboxylated osteocalcin (cOcn) to assess bone mass changes and their relationships with the ratios of ucOcn-to-cOcn in mice from different age groups. The results showed that mouse trabecular bone mass reduced gradually with age, whereas cortical bone loss and mechanical property changes mostly occurred in advanced age. Our findings further demonstrated that the increase in osteoclast activity and the decrease in osteoblast function were significantly corelated with blood levels of ucOcn and cOcn, respectively. The dynamic metabolic changes of ucOcn to cOcn ratio were correlated with age-dependent bone loss in mice. In summary, metabolic shifts in the ratio of ucOcn to cOcn toward bone resorption from young adult to elderly mice contribute to the pathogenesis of age-related bone loss. Simultaneously monitoring blood ratios of ucOcn-to-cOcn may be useful to predict the status of bone mass in vivo.NEW & NOTEWORTHY To our knowledge, our finding in this study shows for the first time that metabolic shifts in ratio of ucOcn to cOcn toward bone resorption are markedly correlated with age-dependent bone loss in male mice. These findings for the effects of aging on bone loss will assist in studying the pathogenesis of human type II osteoporosis.

Wnt7b overexpression in osteoblasts stimulates bone formation and reduces obesity in mice on a high-fat diet

Previous studies have shown that Wnt7b potently stimulates bone formation by promoting osteoblast differentiation and activity. As high-fat feeding leads to obesity and systemic metabolic dysregulation, here we investigate the potential benefit of Wnt7b overexpression in osteoblasts on both bone and whole-body metabolism in mice fed with a high-fat diet (HFD). Wnt7b overexpression elicited massive overgrowth of trabecular and cortical bone but seemed to ameliorate body fat accumulation in mice with prolonged HFD feeding. In addition, Wnt7b overexpression modestly improved glucose tolerance in male mice on HFD. Collectively, the results indicate that targeted overexpression of Wnt7b in osteoblasts not only stimulates bone formation but also improves certain aspects of global metabolism in overnourished mice.

Osteocalcin and Its Potential Functions for Preventing Fatty Liver Hemorrhagic Syndrome in Poultry

Osteocalcin (OCN) is synthesized and secreted by differentiating osteoblasts. In addition to its role in bone, OCN acts as a hormone in the pancreas, liver, muscle, fat, and other organs to regulate multiple pathophysiological processes including glucose homeostasis and adipic acid metabolism. Fat metabolic disorder, such as excessive fat buildup, is related to non-alcoholic fatty liver disease (NAFLD) in humans. Similarly, fatty liver hemorrhage syndrome (FLHS) is a metabolic disease in laying hens, resulting from lipid accumulation in hepatocytes. FLHS affects hen health with significant impact on poultry egg production. Many studies have proposed that OCN has protective function in mammalian NAFLD, but its function in chicken FLHS and related mechanism have not been completely clarified. Recently, we have revealed that OCN prevents laying hens from FLHS through regulating the JNK pathway, and some pathways related to the disease progression have been identified through both in vivo and vitro investigations. In this view, we discussed the current findings for predicting the strategy for using OCN to prevent or reduce FLHS impact on poultry production.

Assessment of Uncarboxylated Osteocalcin Levels in Type 2 Diabetes Mellitus

Osteocalcin is one of the main organic components of the bone matrix and consists of 49 amino acids excreted from osteoblastic cells in carboxylated and uncarboxylated forms. Carboxylated Osteocalcin belongs to the bone matrix, whereas uncarboxylated osteocalcin (ucOC) is an important enzyme of osteocalcin in the circulatory system. It is an essential protein for balancing the minerals in bones, binding with calcium, and regulating body glucose levels. In this review, we point out the assessment of ucOC levels in type 2 diabetes mellitus. The experimental results that show ucOC controls glucose metabolism are significant because they relate to the current obesity, diabetes, and cardiovascular disease. To confirm that, low serum levels of ucOC were a risk factor for poor glucose metabolism, and further clinical studies are required.

Undercarboxylated, but not Carboxylated, Osteocalcin suppresses TNF-α induced inflammatory signaling pathway in Myoblast

Undercarboxylated osteocalcin (ucOCN) has been considered to be an important endocrine factor, especially to regulate bone and energy metabolism. Even with the mounting evidence showing the consistent inverse correlation of ucOCN levels in chronic inflammatory diseases, however, the mechanism underlying the involvement of ucOCN in the muscular inflammation has not been fully understood. In the present study, we explored 1) the endocrine role of ucOCN in the regulation of inflammation in C2C12 myoblasts and primary myoblasts and the underlying intracellular signaling mechanisms, and 2) whether G protein–coupled receptor family C group 6 member A (GPRC6A) is the ucOCN-sensing receptor associated with the ucOCN-mediated anti-inflammatory signaling pathway in myoblasts. ucOCN suppressed the tumor necrosis factor-α (TNF-α)–induced expressions of major inflammatory cytokines, including interleukin-1β (IL-1β) and inhibited the TNF-α–stimulated activities of transcription factors, including NF-κB, in C2C12 and primary myoblasts. Both knockdown and knockout of GPRC6A, by using siRNA or a CRISPR/CAS9 system, respectively, did not reverse the effect of ucOCN on IL-1β expression in myoblasts. Interestingly, TNF-α–induced IL-1β expression was inhibited by knockdown or deletion of GPRC6A itself, regardless of the ucOCN treatment. ucOCN was rapidly internalized into the cytoplasmic region via caveolae-mediated endocytosis, suggesting the presence of new target proteins in the cell membrane and/or in the cytoplasm for interaction with ucOCN in myoblasts. Taken together, these findings indicate that ucOCN suppresses the TNF-α–induced inflammatory signaling pathway in myoblasts. GPRC6A is not a sensing receptor associated with the ucOCN-mediated anti-inflammatory signaling pathway in myoblasts.

Embryonic osteocalcin signalling determines lifelong adrenal steroidogenesis and homeostasis in the mouse

Through their ability to regulate gene expression in most organs, glucocorticoid hormones influence numerous physiological processes and therefore are key regulators of organismal homeostasis. In bone, glucocorticoid hormones inhibit the expression of the hormone Osteocalcin for poorly understood reasons. Here we show that in a classical endocrine feedback loop, osteocalcin in return enhances the biosynthesis of glucocorticoid but also mineralocorticoid hormones (adrenal steroidogenesis) in rodents and primates. Conversely, inactivating osteocalcin signalling in adrenal glands significantly impairs adrenal growth and steroidogenesis in mice. Embryo-made osteocalcin is necessary for normal Sf1 expression in foetal adrenal cells and adrenal cell steroidogenic differentiation, it therefore determines the number of steroidogenic cells present in adrenal glands of adult animals. Embryonic not postnatal osteocalcin also governs adrenal growth, adrenal steroidogenesis, blood pressure, electrolyte equilibrium and the rise of circulating corticosterone during the acute stress response in adult offspring. This osteocalcin-dependent regulation of adrenal development and steroidogenesis occurs even in the absence of a functional of hypothalamus-pituitary-adrenal axis; this explains why osteocalcin administration during pregnancy promotes adrenal growth and steroidogenesis and improves survival of adrenocorticotropic hormone signalling-deficient animals. This study reveals that a bone-derived, embryonic hormone influences lifelong adrenal functions and organismal homeostasis in the mouse.

Advances in bone turnover markers

Bone fragility fractures remain an important worldwide health and economic problem due to increased morbidity and mortality. The current methods for predicting fractures are largely based on the measurement of bone mineral density and the utilization of mathematical risk calculators based on clinical risk factors for bone fragility. Despite these approaches, many bone fractures remain undiagnosed. Therefore, current research is focused on the identification of new factors such as bone turnover markers (BTM) for risk calculation. BTM are a group of proteins and peptides released during bone remodeling that can be found in serum or urine. They derive from bone resorptive and formative processes mediated by osteoclasts and osteoblasts, respectively. Potential use of BTM in monitoring these phenomenon and therefore bone fracture risk is limited by physiologic and pathophysiologic factors that influence BTM. These limitations in predicting fractures explain why their inclusion in clinical guidelines remains limited despite the large number of studies examining BTM.

The cross-talk between matrix metalloproteinase-9, RANKL/OPG system and cardiovascular risk factors in ovariectomized rat model of postmenopausal osteoporosis

Epidemiology and pathogenesis of cardiovascular diseases (CVD) and osteoporosis are strikingly overlapping. This study presents matrix metalloproteinase-9 (MMP-9), as a simple molecular link more consistently associated with the pathophysiology of both osteoporosis and CVD risk factors. 40 adult female rats were randomly distributed into 4 groups [control sham-operated, untreated osteoporosis, carvedilol-treated osteoporosis and alendronate-treated osteoporosis]. After 8 weeks, blood samples were collected to estimate Lipid profile (Total cholesterol, HDL, Triglycerides), inflammatory markers (IL-6, TNF alpha, CRP and NO), and Bone turnover markers (BTM) (Alkaline phosphatase, osteocalcin and pyridinoline). The tibias were dissected to estimate MMP-9 and NF-kB gene expression, OPG, RANKL levels and for histological examination. Induction of osteoporosis resulted in a significant elevation in BTM, inflammatory markers and dyslipidemia. MMP-9 was significantly elevated and positively correlated with BTM, inflammation and dyslipidemia markers. Carvedilol and alendronate exerted a bone preservative role and attenuated dyslipidaemia and inflammation in accordance with their respective effect on MMP-9.

Gain-of-Function Lrp5 Mutation Improves Bone Mass and Strength and Delays Hyperglycemia in a Mouse Model of Insulin-Deficient Diabetes

High fracture rate and high circulating levels of the Wnt inhibitor, sclerostin, have been reported in diabetic patients. We studied the effects of Wnt signaling activation on bone health in a mouse model of insulin-deficient diabetes. We introduced the sclerostin-resistant Lrp5^A214V mutation, associated with high bone mass, in mice carrying the Ins2^Akita mutation (Akita), which results in loss of beta cells, insulin deficiency, and diabetes in males. Akita mice accrue less trabecular bone mass with age relative to wild type (WT). Double heterozygous Lrp5^A214V /Akita mutants have high trabecular bone mass and cortical thickness relative to WT animals, as do Lrp5^A214V single mutants. Likewise, the Lrp5^A214V mutation prevents deterioration of biomechanical properties occurring in Akita mice. Notably, Lrp5^A214V /Akita mice develop fasting hyperglycemia and glucose intolerance with a delay relative to Akita mice (7 to 8 vs. 5 to 6 weeks, respectively), despite lack of insulin production in both groups by 6 weeks of age. Although insulin sensitivity is partially preserved in double heterozygous Lrp5^A214V /Akita relative to Akita mutants up to 30 weeks of age, insulin-dependent phosphorylated protein kinase B (pAKT) activation in vitro is not altered by the Lrp5^A214V mutation. Although white adipose tissue depots are equally reduced in both compound and Akita mice, the Lrp5^A214V mutation prevents brown adipose tissue whitening that occurs in Akita mice. Thus, hyperactivation of Lrp5-dependent signaling fully protects bone mass and strength in prolonged hyperglycemia and improves peripheral glucose metabolism in an insulin independent manner. Wnt signaling activation represents an ideal therapeutic approach for diabetic patients at high risk of fracture. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Male but not female mice with severe osteogenesis imperfecta are partially protected from high-fat diet-induced obesity

Previously we have shown that young mice with a dominant severe form of osteogenesis imperfecta (OI), caused by mutated collagen type I, exhibit an altered glucose/insulin metabolism and energy expenditure along with elevated levels of osteocalcin, a bone-derived hormone involved in the regulation of whole-body metabolism. This study aimed to examine the long-term effects of a western diet in these OI mice. Male and female OI mice and wild type littermates (WT) were fed a high-fat diet (HFD) or a matched low-fat diet (LFD) for 26 weeks. HFD-induced obesity was observed in male and female WT and female OI mice, but not in male OI mice. HFD-fed WT and OI mice of both sexes developed hyperglycemia and glucose intolerance, but the degree of glucose intolerance was significantly lower in male and female OI mice compared to sex- and diet-matched WT mice. Indirect calorimetry revealed increased movement of male OI mice on HFD compared to LFD and, while HFD lowered energy expenditure in WT mice, energy expenditure was not changed in OI mice. Further, HFD-fed male OI mice demonstrated a diet-induced increased expression of the thermogenesis genes, Ucp1 and Pgc1α, in brown adipose tissue. On LFD, total and Gla-13 osteocalcin levels were similar in 30-week-old WT and OI mice, but on HFD, both were significantly higher in OI mice than WT. Thus, male OI mice respond to HFD with increased movement, energy expenditure, brown adipose tissue thermogenesis, and higher levels of osteocalcin, resulting in partial protection against HFD-induced obesity.

Osteoblast-specific deficiency of ectonucleotide pyrophosphatase or phosphodiesterase-1 engenders insulin resistance in high-fat diet fed mice

Supraphysiological levels of the osteoblast-enriched mineralization regulator ectonucleotide pyrophosphatase or phosphodiesterase-1 (NPP1) is associated with type 2 diabetes mellitus. We determined the impact of osteoblast-specific Enpp1 ablation on skeletal structure and metabolic phenotype in mice. Female, but not male, 6-week-old mice lacking osteoblast NPP1 expression (osteoblast-specific knockout [KO]) exhibited increased femoral bone volume or total volume (17.50% vs. 11.67%; p < .01), and reduced trabecular spacing (0.187 vs. 0.157 mm; p < .01) compared with floxed (control) mice. Furthermore, an enhanced ability of isolated osteoblasts from the osteoblast-specific KO to calcify their matrix in vitro compared to fl/fl osteoblasts was observed (p < .05). Male osteoblast-specific KO and fl/fl mice showed comparable glucose and insulin tolerance despite increased levels of insulin-sensitizing under-carboxylated osteocalcin (195% increase; p < .05). However, following high-fat-diet challenge, osteoblast-specific KO mice showed impaired glucose and insulin tolerance compared with fl/fl mice. These data highlight a crucial local role for osteoblast NPP1 in skeletal development and a secondary metabolic impact that predominantly maintains insulin sensitivity.

Skeleton-vasculature chain reaction: a novel insight into the mystery of homeostasis

Angiogenesis and osteogenesis are coupled. However, the cellular and molecular regulation of these processes remains to be further investigated. Both tissues have recently been recognized as endocrine organs, which has stimulated research interest in the screening and functional identification of novel paracrine factors from both tissues. This review aims to elaborate on the novelty and significance of endocrine regulatory loops between bone and the vasculature. In addition, research progress related to the bone vasculature, vessel-related skeletal diseases, pathological conditions, and angiogenesis-targeted therapeutic strategies are also summarized. With respect to future perspectives, new techniques such as single-cell sequencing, which can be used to show the cellular diversity and plasticity of both tissues, are facilitating progress in this field. Moreover, extracellular vesicle-mediated nuclear acid communication deserves further investigation. In conclusion, a deeper understanding of the cellular and molecular regulation of angiogenesis and osteogenesis coupling may offer an opportunity to identify new therapeutic targets.

Undercarboxylated osteocalcin inhibits the early differentiation of osteoclast mediated by Gprc6a

Osteocalcin (OCN) was the most abundant noncollagen protein and considered as an endocrine factor. However, the functions of Undercarboxylated osteocalcin (ucOCN) on osteoclast and bone resorption are not well understood. In the present study, preosteoclast RAW264.7 cells and bone marrow mononuclear cells (BMMs) were treated with ucOCN purified from prokaryotic bacteria. Our results showed that ucOCN attenuated the proliferation of RAW264.7 cells with a concentration dependant manner by MTS assay. Scrape wounding assay revealed the decreased motility of RAW264.7 cells after ucOCN treatment. RT-qPCR results manifested the inhibitory effects of ucOCN on the expression of osteoclastic marker genes in RAW264.7 cells during inducing differentiation of RANKL. It was also observed that ucOCN inhibited the formation of multinucleated cells from RAW264.7 cells and BMMs detected by TRAP staining. The number and area of bone resorb pits were also decreased after treatment with ucOCN during their osteoclast induction by toluidine blue staining. The formation and integrity of the osteoclast actin ring were impaired by ucOCN by immunofluorescent staining. Time dependant treatment of ucOCN during osteoclastic induction demonstrated the inhibitory effects mainly occurred at the early stage of osteoclastogenesis. Signaling analysis of luciferase activity of the CRE or SRE reporter and ERK1/2 phosphorylation showed the selective inhibitor or siRNA of Gprc6a (a presumptive ucOCN receptor) could attenuate the promotion of ucOCN on CRE-luciferase activity. Taken together, we provided the first evidence that ucOCN had negative effects on the early differentiation and bone resorption of osteoclasts via Gprc6a.

Association between Serum Osteocalcin Levels and Metabolic Syndrome according to the Menopausal Status of Korean Women

BACKGROUND

Osteocalcin is known to regulate energy metabolism. Recently, metabolic syndrome (MetS) has been found to be associated with reduced levels of osteocalcin in men, as well as in postmenopausal women. The aim of this study was to investigate the association between serum osteocalcin and MetS in premenopausal women, compared with that in postmenopausal women.

METHODS

This cross-sectional study was based on 5,896 participants who completed a health screening examination. They were classified according to their menopausal status. Each group was subdivided into non-MetS and MetS groups according to the modified National Cholesterol Education Program-Adult Treatment Panel III criteria. Serum osteocalcin levels were measured using the electrochemiluminescence immunoassay.

RESULTS

Serum osteocalcin level was significantly lower in women with MetS than in those without MetS, after adjusting for confounders (14.12 ± 0.04 vs. 13.17 ± 0.13 [P = 0.004] in premenopausal women, and 20.34 ± 0.09 vs. 19.62 ± 0.21 [P < 0.001] in postmenopausal women), regardless of their menopausal status. Serum osteocalcin levels decreased correspondingly with an increasing number of MetS elements (P for trend < 0.001). Multiple regression analysis demonstrated that waist circumference (β = -0.085 [P < 0.001] and β = -0.137 [P < 0.001]) and hemoglobin A1c (β = -0.09 [P < 0.001] and β = -0.145 [P < 0.001]) were independent predictors of osteocalcin in premenopausal and postmenopausal women. Triglyceride levels were also independently associated with osteocalcin levels in premenopausal women (β = -0.004 [P < 0.013]). The odds ratio (OR) for MetS was significantly higher in the lowest quartile than in the highest quartile of serum osteocalcin levels after adjusting for age, alkaline phosphatase, uric acid, high sensitivity C-reactive protein, and body mass index in all women (OR, 2.00; 95% confidence interval [CI], 1.49-2.68) as well as in premenopausal (OR, 2.23; 95% CI, 1.39-3.58) and postmenopausal (OR, 2.01; 95% CI, 1.26-3.23) subgroups.

CONCLUSION

Lower serum osteocalcin concentrations were significantly associated with MetS in both premenopausal and postmenopausal women and were therefore independent of menopausal status.

Matrix metalloproteinase 9 a potential major player connecting atherosclerosis and osteoporosis in high fat diet fed rats

Background

Cardiovascular diseases (CVD) represent one of the major sequelae of obesity. On the other hand, the relationship between bone diseases and obesity remains unclear. An increasing number of biological and epidemiological studies suggest the presence of a link between atherosclerosis and osteoporosis, however, the precise molecular pathways underlying this close association remain poorly understood. The present work thus aimed to study Matrix Metalloproteinase 9 (MMP-9), as a proposed link between atherosclerosis and osteoporosis in high fat diet fed rats.

Methods and findings

40 rats were randomly divided into 4 groups: control, untreated atherosclerosis group, atherosclerotic rats treated with carvedilol (10mg/kg/d) and atherosclerotic rats treated with alendronate sodium (10mg/kg/d). After 8 weeks, blood samples were collected for estimation of Lipid profile (Total cholesterol, HDL, TGs), inflammatory markers (IL-6, TNF-α, CRP and NO) and Bone turnover markers (BTMs) (Alkaline phosphatase, osteocalcin and pyridinoline). Rats were then euthanized and the aortas and tibias were dissected for histological examination and estimation of MMP-9, N-terminal propeptide of type I procollagen (PINP), C-terminal telopeptide of type I collagen (CTX) and NF-kB expression. Induction of atherosclerosis via high fat diet and chronic stress induced a significant increase in BTMs, inflammatory markers and resulted in a state of dyslipidaemia. MMP-9 has also shown to be significantly increased in the untreated atherosclerosis rats and showed a significant correlation with all measured parameters. Interestingly, Carvedilol and bisphosphonate had almost equal effects restoring the measured parameters back to normal, partially or completely.

Conclusion

MMP-9 is a pivotal molecule that impact the atherogenic environment of the vessel wall. A strong cross talk exists between MMP-9, cytokine production and macrophage function. It also plays an important regulatory role in osteoclastogenesis. So, it may be a key molecule in charge for coupling CVD and bone diseases in high fat diet fed rats. Therefore, we suggest MMP-9 as a worthy molecule to be targeted pharmacologically in order to control both conditions simultaneously. Further studies are needed to support, to invest and to translate this hypothesis into clinical studies and guidelines.

Understanding the Neurophysiological and Molecular Mechanisms of Exercise-Induced Neuroplasticity in Cortical and Descending Motor Pathways: Where Do We Stand?

Exercise is a promising, cost-effective intervention to augment successful aging and neurorehabilitation. Decline of gray and white matter accompanies physiological aging and contributes to motor deficits in older adults. Exercise is believed to reduce atrophy within the motor system and induce neuroplasticity which, in turn, helps preserve motor function during aging and promote re-learning of motor skills, for example after stroke. To fully exploit the benefits of exercise, it is crucial to gain a greater understanding of the neurophysiological and molecular mechanisms underlying exercise-induced brain changes that prime neuroplasticity and thus contribute to postponing, slowing, and ameliorating age- and disease-related impairments in motor function. This knowledge will allow us to develop more effective, personalized exercise protocols that meet individual needs, thereby increasing the utility of exercise strategies in clinical and non-clinical settings. Here, we review findings from studies that investigated neurophysiological and molecular changes associated with acute or long-term exercise in healthy, young adults and in healthy, postmenopausal women.

The half-life of the bone-derived hormone osteocalcin is regulated through O-glycosylation in mice, but not in humans

Osteocalcin (OCN) is an osteoblast-derived hormone with pleiotropic physiological functions. Like many peptide hormones, OCN is subjected to post-translational modifications (PTMs) which control its activity. Here, we uncover O-glycosylation as a novel PTM present on mouse OCN and occurring on a single serine (S8) independently of its carboxylation and endoproteolysis, two other PTMs regulating this hormone. We also show that O-glycosylation increases OCN half-life in plasma ex vivo and in the circulation in vivo. Remarkably, in human OCN (hOCN), the residue corresponding to S8 is a tyrosine (Y12), which is not O-glycosylated. Yet, the Y12S mutation is sufficient to O-glycosylate hOCN and to increase its half-life in plasma compared to wildtype hOCN. These findings reveal an important species difference in OCN regulation, which may explain why serum concentrations of OCN are higher in mouse than in human.

Bones provide support and protection for organs in the body. However, over the last 15 years researchers have discovered that bones also release chemicals known as hormones, which can travel to other parts of the body and cause an effect. The cells responsible for making bone, known as osteoblasts, produce a hormone called osteocalcin which communicates with a number of different organs, including the pancreas and brain.

When osteocalcin reaches the pancreas, it promotes the release of another hormone called insulin which helps regulate the levels of sugar in the blood. Osteocalcin also travels to other organs such as muscle, where it helps to degrade fats and sugars that can be converted into energy. It also has beneficial effects on the brain, and has been shown to aid memory and reduce depression.

Osteocalcin has largely been studied in mice where levels are five to ten times higher than in humans. But it is unclear why this difference exists or how it alters the role of osteocalcin in humans. To answer this question, Al Rifai et al. used a range of experimental techniques to compare the structure and activity of osteocalcin in mice and humans.

The experiments showed that mouse osteocalcin has a group of sugars attached to its protein structure, which prevent the hormone from being degraded by an enzyme in the blood. Human osteocalcin has a slightly different protein sequence and is therefore unable to bind to this sugar group. As a result, the osteocalcin molecules in humans are less stable and cannot last as long in the blood. Al Rifai et al. showed that when human osteocalcin was modified so the sugar group could attach, the hormone was able to stick around for much longer and reach higher levels when added to blood in the laboratory.

These findings show how osteocalcin differs between human and mice. Understanding this difference is important as the effects of osteocalcin mean this hormone can be used to treat diabetes and brain disorders. Furthermore, the results reveal how the stability of osteocalcin could be improved in humans, which could potentially enhance its therapeutic effect.

Recombinant Antibodies with Unique Specificities Allow for Sensitive and Specific Detection of Uncarboxylated Osteocalcin in Human Circulation

Osteocalcin is a bone-specific protein which contains three glutamic acid residues (Glu) that undergo post-translational gamma-carboxylation. Uncarboxylated osteocalcin (ucOC) may participate in the regulation of glucose metabolism, thus measurement of ucOC could be useful in evaluating interactions between bone and glucose metabolism. We developed recombinant antibodies and immunoassay to specifically detect ucOC in human blood samples. ucOC-specific recombinant antibodies were selected from an antibody library by phage display. Four candidates were characterized, and one (Fab-AP13) was used to set up an immunoassay with a pre-existing MAb. Plasma ucOC levels were measured in subjects with normal fasting blood glucose (≤ 6 mmol/l, N = 46) or with hyperglycemia (≥ 7 mmol/l, N = 29). Further, we analyzed ucOC in age- and gender-matched patients with diagnosed type 2 diabetes (T2D, N = 49). Antibodies recognized ucOC without cross-reaction to carboxylated osteocalcin. Antibodies had unique binding sites at the carboxylation region, with Glu17 included in all epitopes. Immunoassay was set up and characterized. Immunoassay detected ucOC in serum and plasma, with on average 1.6-fold higher levels in plasma. ucOC concentrations were significantly lower in subjects with hyperglycemia (median 0.58 ng/ml, p = 0.008) or with T2D diagnosis (0.68 ng/ml, p = 0.015) than in subjects with normal blood glucose (1.01 ng/ml). ucOC negatively correlated with fasting plasma glucose in subjects without T2D (r = - 0.24, p = 0.035) but not in T2D patients (p = 0.41). Our immunoassay, based on the novel recombinant antibody, allows for specific and sensitive detection of ucOC in human circulation. Correlation between ucOC and plasma glucose suggests interactions between osteocalcin and glucose metabolism in humans.

Comparative analysis of titanium coating on cobalt-chrome alloy in vitro and in vivo direct metal fabrication vs. plasma spraying

Background

Titanium surface coating on cobalt-chromium (CoCr) alloy has characteristics desirable for an orthopedic implant as follows: strength, osteointegrative capability, and biocompatibility. Creating such a coated surface takes a challenging process and two dissimilar metals are not easily welded. In our study, we utilized additive manufacturing with a 3D printing called direct metal fabrication (DMF) and compared it to the plasma spraying method (TPS), to coat titanium onto CoCr alloy. We hypothesized that this would yield a coated surface quality as acceptable or better than the already established method of plasma spraying. For this, we compared characteristics of titanium-coated surfaces created by direct metal fabrication method (DMF) and titanium plasma spraying (TPS), both in vitro and in vivo, for (1) cell morphology, (2) confocal microscopy images of immunofluorescent assay of RUNX2 and fibronectin, (3) quantification of cell proliferation rate, (4) push-out biomechanical test, and (5) bone histomorphometry.

Method

For in vitro study, human osteoblast cells were seeded onto the coated surfaces. Cellular morphology was observed with a scanning electron microscope. Cellular proliferation was validated with ELISA, immunofluorescent assay. For in vivo study, coated rods were inserted into the distal femur of the rabbit and then harvested. The rods were biomechanically tested with a push-out test and observed for histomorphometry to evaluate the microscopic bone to implant ratio.

Result

For cell morphology observation, lamellipodia and filopodia, a cytoplasmic projection extending into porous structure, formed on both surfaces created by DMF and TPS. The proliferation of the osteoblasts, the DMF group showed a better result at different optic density levels (p = 0.035, 0.005, 0.001). Expression and distribution of fibronectin and Runx-2 genes showed similar degrees of expressions. The biomechanical push-out test yielded a similar result (p = 0.714). Histomorphometry analysis also showed a similar result (p = 0.657).

Conclusion

In conclusion, DMF is a method which can reliably create a proper titanium surface on CoCr alloy. The resulting product of the surface shows a similar quality to that of the plasma spraying method, both in vivo and in vitro, in terms of biological and mechanical property.

Aptamer selection and aptasensor construction for bone density biomarkers

Osteoporosis (OP) is a bone disease involved in dysregulation of one of the bone metabolism arms, formation, or desorption cause a porous bone. Osteocalcin (OC) and beta-crosslap (BC), are the well-known markers for OP, which are connected to bone formation and desorption, respectively. In addition to the OP biomarker, BC is also used as an estrogen replacement therapeutic monitoring. ELISA and other antibody-based detection methods are routinely used for measuring OC and BC. These methods have limitations that include thermostability, sensitivity, sacrificing animals, and cost of production. However, aptamer-based-assays are of interest to overcome these drawbacks and achieve the most specific and robust application. Herein, specific aptamers for OC and BC were selected by the systematic evolution of ligands by exponential enrichment (SELEX) method from the pool of ssDNA library with 60 random sequences. The binding affinity (K_d) of the selected aptamers were evaluated against the respective biomarkers. The high-affinity aptamers of OC and BC showed the K_d values of 59 and 55 nM respectively. A graphene oxide-based aptasensors were fabricated from the high-affinity aptamers, and the detection limits of OC and BC were found to be 0.4 pg/ml and 0.21 pg/ml, respectively. These aptasensors have been tested with OC and BC spiked buffer samples and validated using serum samples collected from osteoporotic rats.

PHOSPHO1 is a skeletal regulator of insulin resistance and obesity

BACKGROUND

The classical functions of the skeleton encompass locomotion, protection and mineral homeostasis. However, cell-specific gene deletions in the mouse and human genetic studies have identified the skeleton as a key endocrine regulator of metabolism. The bone-specific phosphatase, Phosphatase, Orphan 1 (PHOSPHO1), which is indispensable for bone mineralisation, has been recently implicated in the regulation of energy metabolism in humans, but its role in systemic metabolism remains unclear. Here, we probe the mechanism underlying metabolic regulation by analysing Phospho1 mutant mice.

RESULTS

Phospho1^-/- mice exhibited improved basal glucose homeostasis and resisted high-fat-diet-induced weight gain and diabetes. The metabolic protection in Phospho1^-/- mice was manifested in the absence of altered levels of osteocalcin. Osteoblasts isolated from Phospho1^-/- mice were enriched for genes associated with energy metabolism and diabetes; Phospho1 both directly and indirectly interacted with genes associated with glucose transport and insulin receptor signalling. Canonical thermogenesis via brown adipose tissue did not underlie the metabolic protection observed in adult Phospho1^-/- mice. However, the decreased serum choline levels in Phospho1^-/- mice were normalised by feeding a 2% choline rich diet resulting in a normalisation in insulin sensitivity and fat mass.

CONCLUSION

We show that mice lacking the bone mineralisation enzyme PHOSPHO1 exhibit improved basal glucose homeostasis and resist high-fat-diet-induced weight gain and diabetes. This study identifies PHOSPHO1 as a potential bone-derived therapeutic target for the treatment of obesity and diabetes.

Osteocalcin prevents insulin resistance, hepatic inflammation, and activates autophagy associated with high-fat diet-induced fatty liver hemorrhagic syndrome in aged laying hens

The aim of this study was to investigate the effects of osteocalcin (OCN) on fatty liver hemorrhagic syndrome (FLHS) in aged laying hens. Thirty 68-week-old White Plymouth laying hens were randomly assigned into conventional single-bird cages, and the cages were randomly allocated into one of 3 treatments (n = 10): normal diet (ND + vehicle, ND + V), high-fat diet (HFD + vehicle, HFD + V), and HFD + OCN (3 μg/bird, 1 time/2 d, i.m.) for 40 d. At day 30, oral glucose tolerance tests (OGTT) and insulin tolerance tests (ITT) were performed. At the end of experiment, the hens were euthanized followed by blood collection. The plasma aspartate transaminase (AST), alkaline phosphatase (ALP), total cholesterol (TC), triglyceride (TG), low-density lipoprotein cholesterol (LDL-C), and high-density lipoprotein cholesterol (HDL-C) were measured using an automatic biochemistry analyzer. Pathological changes in the liver were examined under both light and transmission electron microscopes. The plasma inflammatory factors including interleukin-1 (IL-1), IL-6, and tumor necrosis factor-alpha (TNF-α) were analyzed by ELISA, and the gene expressions of these inflammatory factors in the liver were analyzed by real-time PCR. The level of oxidative stress was evaluated using malondialdehyde (MDA) and glutathione peroxidase (GSH-Px) assay kits, respectively. The results showed that HFD + V hens had more severe liver hemorrhage and fibrosis than ND + V hens (P < 0.05). The ultramicrostructural examination showed that hepatocytes of HFD + V hens exhibited necrotic pyknosis showing great intracellular electron, mitochondrial swelling, shrunk nucleus, and absence of autolysosomes. Osteocalcin mitigated HFD + V-induced pathological changes in aged laying hens. High-fat diet + OCN hens had higher insulin sensitivity; lower liver concentrations of MDA (P = 0.12) but higher GSH-Px (P < 0.05); and lower blood TNF-α concentrations (P < 0.05) and mRNA expressions (P < 0.05) than HFD + V hens. These results suggest OCN functions in preventing the FLHS process in old laying hens through inhibiting excessive energy diet-induced metabolic disorder, oxidative stress, and related pathological damage.

Bone Regulation of Insulin Secretion and Glucose Homeostasis

For centuries our image of the skeleton has been one of an inert structure playing a supporting role for muscles and a protective role for inner organs like the brain. Cell biology and physiology modified this view in the 20st century by defining the constant interplay between bone-forming and bone resorbing cells that take place during bone growth and remodeling, therefore demonstrating that bone is as alive as any other tissues in the body. During the past 40 years human and, most important, mouse genetics, have allowed not only the refinement of this notion by identifying the many genes and regulatory networks responsible for the crosstalk existing between bone cells, but have redefined the role of bone by showing that its influence goes way beyond its own physiology. Among its newly identified functions is the regulation of energy metabolism by 2 bone-derived hormones, osteocalcin and lipocalin-2. Their biology and respective roles in this process are the topic of this review.

Roles of bone-derived hormones in type 2 diabetes and cardiovascular pathophysiology

Background

Emerging evidence demonstrates that bone is an endocrine organ capable of influencing multiple physiological and pathological processes through the secretion of hormones. Recent research suggests complex crosstalk between the bone and other metabolic and cardiovascular tissues. It was uncovered that three of these bone-derived hormones—osteocalcin, lipocalin 2, and sclerostin—are involved in the endocrine regulations of cardiometabolic health and play vital roles in the pathophysiological process of developing cardiometabolic syndromes such as type 2 diabetes and cardiovascular disease. Chronic low-grade inflammation is one of the hallmarks of cardiometabolic diseases and a major contributor to disease progression. Novel evidence also implicates important roles of bone-derived hormones in the regulation of chronic inflammation.

Scope of review

In this review, we provide a detailed overview of the physiological and pathological roles of osteocalcin, lipocalin 2, and sclerostin in cardiometabolic health regulation and disease development, with a focus on the modulation of chronic inflammation.

Major conclusions

Evidence supports that osteocalcin has a protective role in cardiometabolic health, and an increase of lipocalin 2 contributes to the development of cardiometabolic diseases partly via pro-inflammatory effects. The roles of sclerostin appear to be complicated: It exerts pro-adiposity and pro-insulin resistance effects in type 2 diabetes and has an anti-calcification effect during cardiovascular disease. A better understanding of the actions of these bone-derived hormones in the pathophysiology of cardiometabolic diseases will provide crucial insights to help further research develop new therapeutic strategies to treat these diseases.

Biomarkers of vascular calcification in serum

Over the last decades, the association between vascular calcification (VC) and all-cause/cardiovascular mortality, especially in patients with high atherogenic status, such as those with diabetes and/or chronic kidney disease, has been repeatedly highlighted. For over a century, VC has been noted as a passive, degenerative, aging process without any treatment options. However, during the past decades, studies confirmed that mineralization of the arteries is an active, complex process, similar to bone genesis and formation. The main purpose of this review is to provide an update of the existing biomarkers of VC in serum and develop the various pathogenetic mechanisms underlying the calcification process, including the pivotal roles of matrix Gla protein, osteoprotegerin, bone morphogenetic proteins, fetuin-a, fibroblast growth-factor-23, osteocalcin, osteopontin, osteonectin, sclerostin, pyrophosphate, Smads, fibrillin-1 and carbonic anhydrase II.

Measurement of bioactive osteocalcin in humans using a novel immunoassay reveals association with glucose metabolism and β-cell function

Osteocalcin (OCN) is a bone-derived hormone involved in the regulation of glucose metabolism. In serum, OCN exists in carboxylated and uncarboxylated forms (ucOCN), and studies in rodents suggest that ucOCN is the bioactive form of this hormone. Whether this is also the case in humans is unclear, because a reliable assay to measure ucOCN is not available. Here, we established and validated a new immunoassay (ELISA) measuring human ucOCN and used it to determine the level of bioactive OCN in two cohorts of overweight or obese subjects, with or without type 2 diabetes (T2D). The ELISA could specifically detect ucOCN concentrations ranging from 0.037 to 1.8 ng/mL. In a first cohort of overweight or obese postmenopausal women without diabetes (n = 132), ucOCN correlated negatively with fasting glucose (r = -0.18, P = 0.042) and insulin resistance assessed by the homeostatic model assessment of insulin resistance (r = -0.18, P = 0.038) and positively with insulin sensitivity assessed by a hyperinsulinemic-euglycemic clamp (r = 0.18, P = 0.043) or insulin sensitivity index derived from an oral glucose tolerance test (r = 0.26, P = 0.003). In a second cohort of subjects with severe obesity (n = 16), ucOCN was found to be lower in subjects with T2D compared with those without T2D (2.76 ± 0.38 versus 4.52 ± 0.06 ng/mL, P = 0.009) and to negatively correlate with fasting glucose (r = -0.50, P = 0.046) and glycated hemoglobin (r = -0.57, P = 0.021). Moreover, the subjects with ucOCN levels below 3 ng/mL had a reduced insulin secretion rate during a hyperglycemic clamp (P = 0.03). In conclusion, ucOCN measured with this novel and specific assay is inversely associated with insulin resistance and β-cell dysfunction in humans.

The role of osteoblasts in energy homeostasis

Osteoblasts are specialized mesenchymal cells that synthesize bone matrix and coordinate the mineralization of the skeleton. These cells work in harmony with osteoclasts, which resorb bone, in a continuous cycle that occurs throughout life. The unique function of osteoblasts requires substantial amounts of energy production, particularly during states of new bone formation and remodelling. Over the last 15 years, studies have shown that osteoblasts secrete endocrine factors that integrate the metabolic requirements of bone formation with global energy balance through the regulation of insulin production, feeding behaviour and adipose tissue metabolism. In this article, we summarize the current understanding of three osteoblast-derived metabolic hormones (osteocalcin, lipocalin and sclerostin) and the clinical evidence that suggests the relevance of these pathways in humans, while also discussing the necessity of specific energy substrates (glucose, fatty acids and amino acids) to fuel bone formation and promote osteoblast differentiation.

The Potential Role of Undercarboxylated Osteocalcin Upregulation in Microvascular Insufficiency in a Rat Model of Diabetic Cardiomyopathy

BACKGROUND

Diabetic cardiomyopathy (DCM) is accompanied by microvascular complications that lead to myocardial dysfunction and heart failure. Most conventional therapies cannot ameliorate the microvascular insufficiency in DCM. In this study, we tested the hypothesis that undercarboxylated osteocalcin (ucOC) may be a new adjuvant therapy against the progression of DCM and its underlying microvascular pathology.

MATERIALS AND METHODS

Diabetes was induced in Wistar rats with a high-fat diet combined with streptozotocin injections, and ucOC was upregulated after warfarin administration in the treated group. After 8 weeks, cardiac functions were assessed using a Langendorff apparatus. Cardiac tissue samples were also extracted to assess the ucOC receptor and vascular endothelial growth factor (VEGF) for histopathological studies.

RESULTS

Both the systolic and the diastolic dysfunction observed in the DCM group were significantly improved after the increase in ucOC blood levels. Significant improvement in VEGF and CD31 expression after warfarin injection was associated with increased capillary density, neovascularization, and decreased myocardial fibrosis together with the reestablishment of myocardial structural and ultrastructural patterns.

CONCLUSION

Undercarboxylated osteocalcin may have a promising effect in improving microvascular insufficiency and myocardial dysfunction in DCM.

Osteokines and the vasculature: a review of the in vitro effects of osteocalcin, fibroblast growth factor-23 and lipocalin-2

Bone-derived factors that demonstrate extra-skeletal functions, also termed osteokines, are fast becoming a highly interesting and focused area of cross-disciplinary endocrine research. Osteocalcin (OCN), fibroblast growth factor-23 (FGF23) and lipocalin-2 (LCN-2), produced in bone, comprise an important endocrine system that is finely tuned with other organs to ensure homeostatic balance and health. This review aims to evaluate in vitro evidence of the direct involvement of these proteins in vascular cells and whether any causal roles in cardiovascular disease or inflammation can be supported. PubMed, Medline, Embase and Google Scholar were searched for relevant research articles investigating the exogenous addition of OCN, FGF23 or LCN-2 to vascular smooth muscle or endothelial cells. Overall, these osteokines are directly vasoactive across a range of human and animal vascular cells. Both OCN and FGF23 have anti-apoptotic properties and increase eNOS phosphorylation and nitric oxide production through Akt signalling in human endothelial cells. OCN improves intracellular insulin signalling and demonstrates protective effects against endoplasmic reticulum stress in murine and human endothelial cells. OCN may be involved in calcification but further research is warranted, while there is no evidence for a pro-calcific effect of FGF23 in vitro. FGF23 and LCN-2 increase proliferation in some cell types and increase and decrease reactive oxygen species generation, respectively. LCN-2 also has anti-apoptotic effects but may increase endoplasmic reticulum stress as well as have pro-inflammatory and pro-angiogenic properties in human vascular endothelial and smooth muscle cells. There is no strong evidence to support a pathological role of OCN or FGF23 in the vasculature based on these findings. In contrast, they may in fact support normal endothelial functioning, vascular homeostasis and vasodilation. No studies examined whether OCN or FGF23 may have a role in vascular inflammation. Limited studies with LCN-2 indicate a pro-inflammatory and possible pathological role in the vasculature but further mechanistic data is required. Overall, these osteokines pose intriguing functions which should be investigated comprehensively to assess their relevance to cardiovascular disease and health in humans.

Circulating osteocalcin as a bone-derived hormone is inversely correlated with body fat in patients with type 1 diabetes

The objective of the present study was to investigate the correlations between serum undercarboxylated osteocalcin (ucOC) or osteocalcin (OC) concentrations and %body fat, serum adiponectin and free-testosterone concentration, muscle strength and dose of exogenous insulin in patients with type 1 diabetes. We recruited 73 Japanese young adult patients with childhood-onset type 1 diabetes. All participants were receiving insulin replacement therapy. The correlations between logarithmic serum ucOC or OC concentrations and each parameter were examined. Serum ucOC and OC concentrations were inversely correlated with %body fat (r = -0.319, P = 0.007; r = -0.321, P = 0.006, respectively). Furthermore, multiple linear regression analyses were performed to determine whether or not serum ucOC or OC concentrations were factors associated with %body fat. Serum ucOC and OC concentrations remained significant factors even after adjusting for gender, HbA1c, body weight-adjusted total daily dose of insulin and duration of diabetes (β = -0.260, P = 0.027; β = -0.254, P = 0.031, respectively). However, serum ucOC and OC concentrations were not correlated with serum adiponectin or free-testosterone concentrations, muscle strength or dose of exogenous insulin. In conclusion, our study demonstrates the inverse correlation between serum ucOC or OC concentrations and body fat in patients with type 1 diabetes.

Serious Hypoglycemia and Use of Warfarin in Combination With Sulfonylureas or Metformin

Prior research suggests that warfarin, when given concomitantly with some sulfonylureas, may increase the risk of serious hypoglycemia. However, the clinical significance remains unclear. We examined rate ratios (RRs) for the association between serious hypoglycemia and concomitant use of warfarin with either sulfonylureas or metformin using a self-controlled case series design and US Medicaid claims (supplemented with Medicare claims) from 1999 to 2011. Across all risk windows combined, warfarin was associated with an elevated rate of serious hypoglycemia when given concomitantly with glimepiride (RR, 1.47; 95% confidence interval (CI), 1.07-2.02) and metformin (RR, 1.73; 95% CI, 1.38-2.16). Particularly in the late risk window (>120 days since beginning concomitancy), most of the RRs for warfarin were elevated: glipizide (RR, 1.72; 95% CI, 1.29-2.29), glyburide (RR, 1.57; 95% CI, 1.15-2.15), metformin (RR, 2.26; 95% CI, 1.67-3.05), and glimepiride (RR, 1.56; 95% CI, 0.97-2.50). These results are consistent with a previously hypothesized hypoglycemic effect of warfarin in patients with type 2 diabetes through inhibition of the carboxylation of osteocalcin.

VKORC1L1, An Enzyme Mediating the Effect of Vitamin K in Liver and Extrahepatic Tissues

Vitamin K is an essential nutrient involved in the regulation of blood clotting and tissue mineralization. Vitamin K oxidoreductase (VKORC1) converts vitamin K epoxide into reduced vitamin K, which acts as the co-factor for the γ-carboxylation of several proteins, including coagulation factors produced by the liver. VKORC1 is also the pharmacological target of warfarin, a widely used anticoagulant. Vertebrates possess a VKORC1 paralog, VKORC1-like 1 (VKORC1L1), but until very recently, the importance of VKORC1L1 for protein γ-carboxylation and hemostasis in vivo was not clear. Here, we first review the current knowledge on the structure, function and expression pattern of VKORC1L1, including recent data establishing that, in the absence of VKORC1, VKORC1L1 can support vitamin K-dependent carboxylation in the liver during the pre- and perinatal periods in vivo. We then provide original data showing that the partial redundancy between VKORC1 and VKORC1L1 also exists in bone around birth. Recent studies indicate that, in vitro and in cell culture models, VKORC1L1 is less sensitive to warfarin than VKORC1. Genetic evidence is presented here, which supports the notion that VKORC1L1 is not the warfarin-resistant vitamin K quinone reductase present in the liver. In summary, although the exact physiological function of VKORC1L1 remains elusive, the latest findings clearly established that this enzyme is a vitamin K oxidoreductase, which can support γ-carboxylation in vivo.

Low serum osteocalcin concentration is associated with incident type 2 diabetes mellitus in Japanese women

Increasing evidence suggests that osteocalcin is involved in the regulation of glucose homeostasis. However, the relationship between serum osteocalcin levels and risk of incident type 2 diabetes mellitus is not clear. The objective of this study is to investigate whether serum osteocalcin levels are associated with the risk of incident type 2 diabetes mellitus. This study included 1691 Japanese postmenopausal women, 61 incident diabetes cases, and 1630 non-diabetic control subjects in the observation period. Baseline concentrations of intact osteocalcin, HbA1c, bone-specific alkaline phosphatase, adiponectin, leptin, urinary N-telopeptides were assessed. Serum osteocalcin levels were significantly correlated with HbA1c levels among 1691 Japanese postmenopausal women (R = -0.12, P < 0.0001). In receiver operating characteristic curve analysis, the optimal cut-off levels for serum osteocalcin to predict the development of type 2 diabetes mellitus was 6.1 ng/mL. The group with baseline osteocalcin levels <6.1 ng/mL showed a significantly higher risk for developing diabetes than the group with baseline osteocalcin levels >6.1 ng/mL (log-rank test, P  <  0.0001) during the mean observation period (7.6 ± 6.1 years; mean ± SD). In multiple Cox proportional hazard analysis, osteocalcin levels were significantly associated with development of type 2 diabetes mellitus during the observation period. Our results indicate that a decrease in serum osteocalcin levels is associated with future development of type 2 diabetes mellitus independent of conventional risk factors in Japanese postmenopausal women.

Undercarboxylated Osteocalcin: Experimental and Human Evidence for a Role in Glucose Homeostasis and Muscle Regulation of Insulin Sensitivity

Recent advances have indicated that osteocalcin, and in particular its undercarboxylated form (ucOC), is not only a nutritional biomarker reflective of vitamin K status and an indicator of bone health but also an active hormone that mediates glucose metabolism in experimental studies. This work has been supported by the putative identification of G protein-coupled receptor, class C, group 6, member A (GPRC6A) as a cell surface receptor for ucOC. Of note, ucOC has been associated with diabetes and with cardiovascular risk in epidemiological studies, consistent with a pathophysiological role for ucOC in vivo. Limitations of existing knowledge include uncertainty regarding the underlying mechanisms by which ucOC interacts with GPRC6A to modulate metabolic and cardiovascular outcomes, technical issues with commonly used assays for ucOC in serum, and a paucity of clinical trials to prove causation and illuminate the scope for novel health interventions. A key emerging area of research is the role of ucOC in relation to expression of GPRC6A in muscle, and whether exercise interventions may modulate metabolic outcomes favorably in part via ucOC. Further research is warranted to clarify potential direct and indirect roles for ucOC in human health and cardiometabolic diseases.

Modulation of cognition and anxiety-like behavior by bone remodeling

Objective

That the bone-derived hormone osteocalcin is necessary to promote normal brain development and function, along with its recently described sufficiency in reversing cognitive manifestations of aging, raises novel questions. One of these is to assess whether bone health, which deteriorates rapidly with aging, is a significant determinant of cognition and anxiety-like behavior.

Methods

To begin addressing this question, we used mice haploinsufficient for Runx2, the master gene of osteoblast differentiation and the main regulator of Osteocalcin expression. Control and Runx2+/− mice were evaluated for the expression of osteocalcin's target genes in the brain and for behavioral parameters, using two assays each for cognition and anxiety-like behavior.

Results

We found that adult Runx2+/− mice had defects in bone resorption, reduced circulating levels of bioactive osteocalcin, and reduced expression of osteocalcin's target genes in the brain. Consequently, they had significant impairment in cognitive function and increased anxiety-like behavior.

Conclusions

These results indicate that bone remodeling is a determinant of brain function.

•

The transcription factor Runx2 determines circulating osteocalcin levels.

•

Runx2 influences cognition through its regulation of osteocalcin expression.

•

Runx2 also influences anxiety-like behavior.

Proprotein convertase furin regulates osteocalcin and bone endocrine function

Osteocalcin (OCN) is an osteoblast-derived hormone that increases energy expenditure, insulin sensitivity, insulin secretion, and glucose tolerance. The cDNA sequence of OCN predicts that, like many other peptide hormones, OCN is first synthesized as a prohormone (pro-OCN). The importance of pro-OCN maturation in regulating OCN and the identity of the endopeptidase responsible for pro-OCN cleavage in osteoblasts are still unknown. Here, we show that the proprotein convertase furin is responsible for pro-OCN maturation in vitro and in vivo. Using pharmacological and genetic experiments, we also determined that furin-mediated pro-OCN cleavage occurred independently of its γ-carboxylation, a posttranslational modification that is known to hamper OCN endocrine action. However, because pro-OCN is not efficiently decarboxylated and activated during bone resorption, inactivation of furin in osteoblasts in mice resulted in decreased circulating levels of undercarboxylated OCN, impaired glucose tolerance, and reduced energy expenditure. Furthermore, we show that Furin deletion in osteoblasts reduced appetite, a function not modulated by OCN, thus suggesting that osteoblasts may secrete additional hormones that regulate different aspects of energy metabolism. Accordingly, the metabolic defects of the mice lacking furin in osteoblasts became more apparent under pair-feeding conditions. These findings identify furin as an important regulator of bone endocrine function.

Osteocalcin-dependent regulation of glucose metabolism and fertility: Skeletal implications for the development of insulin resistance

The skeleton has recently emerged as a critical insulin target tissue that regulates whole body glucose metabolism and male reproductive function. While our understanding of these new regulatory axes remains in its infancy, the bone-specific protein, osteocalcin, has been shown to be centrally involved. Undercarboxylated osteocalcin acts as a secretagogue in a feed-forward loop to stimulate pancreatic β-cell proliferation and insulin secretion, improve insulin sensitivity, and promote testosterone production. Importantly, dysregulation of insulin signaling in bone causes a reduction in serum osteocalcin levels that is associated with elevated blood glucose and reduced serum insulin levels, suggesting that the skeleton may play a significant role in the development of diet-induced insulin resistance. Insulin signaling is negatively regulated by the mammalian target of rapamycin complex 1 (mTORC1) which becomes hyper-activated in response to nutrient overload. Loss- and gain-of function models suggest that mTORC1 function in bone is essential for normal skeletal development; however, the role of this complex in the regulation of glucose metabolism remains to be determined. This review highlights our current understanding of the role played by osteocalcin in the skeletal regulation of glucose metabolism and fertility. In particular, it examines data emerging from transgenic mouse models which have revealed a pancreas-bone-testis regulatory axis and discusses recent human studies which seek to corroborate findings from mouse models with clinical observations. Moreover, we review recent studies which suggest dysregulation of insulin signaling in bone leads to the development of insulin resistance and discuss the potential role of mTORC1 signaling in this process.

Metabolic phenotype in the mouse model of osteogenesis imperfecta

Osteogenesis imperfecta (OI) is the most common heritable bone fragility disorder, usually caused by dominant mutations in genes coding for collagen type I alpha chains, COL1A1 or COL1A2 Osteocalcin (OCN) is now recognized as a bone-derived regulator of insulin secretion and sensitivity and glucose homeostasis. Since OI is associated with increased rates of bone formation and resorption, we hypothesized that the levels of undercarboxylated OCN are increased in OI. The objective of this study was to determine changes in OCN and to elucidate the metabolic phenotype in the Col1a1^Jrt/+ mouse, a model of dominant OI caused by a Col1a1 mutation. Circulating levels of undercarboxylated OCN were higher in 4-week-old OI mice and normal by 8 weeks of age. Young OI animals exhibited a sex-dependent metabolic phenotype, including increased insulin levels in males, improved glucose tolerance in females, lower levels of random glucose and low adiposity in both sexes. The rates of O₂ consumption and CO₂ production, as well as energy expenditure assessed using indirect calorimetry were significantly increased in OI animals of both sexes, whereas respiratory exchange ratio was significantly higher in OI males only. Although OI mice have significant physical impairment that may contribute to metabolic differences, we specifically accounted for movement and compared OI and WT animals during the periods of similar activity levels. Taken together, our data strongly suggest that OI animals have alterations in whole body energy metabolism that are consistent with the action of undercarboxylated osteocalcin.

Gpr158 mediates osteocalcin's regulation of cognition

This study by Khrimian et al. demonstrates that the bone-derived hormone osteocalcin is necessary and sufficient to correct age-related cognitive decline in the mouse. It also provides genetic, molecular, and neurophysiological evidence that Gpr158 is the receptor mediating osteocalcin’s regulation of cognition.

That osteocalcin (OCN) is necessary for hippocampal-dependent memory and to prevent anxiety-like behaviors raises novel questions. One question is to determine whether OCN is also sufficient to improve these behaviors in wild-type mice, when circulating levels of OCN decline as they do with age. Here we show that the presence of OCN is necessary for the beneficial influence of plasma from young mice when injected into older mice on memory and that peripheral delivery of OCN is sufficient to improve memory and decrease anxiety-like behaviors in 16-mo-old mice. A second question is to identify a receptor transducing OCN signal in neurons. Genetic, electrophysiological, molecular, and behavioral assays identify Gpr158, an orphan G protein–coupled receptor expressed in neurons of the CA3 region of the hippocampus, as transducing OCN’s regulation of hippocampal-dependent memory in part through inositol 1,4,5-trisphosphate and brain-derived neurotrophic factor. These results indicate that exogenous OCN can improve hippocampal-dependent memory in mice and identify molecular tools to harness this pathway for therapeutic purposes.

Bone-brain crosstalk and potential associated diseases

Reciprocal relationships between organs are essential to maintain whole body homeostasis. An exciting interplay between two apparently unrelated organs, the bone and the brain, has emerged recently. Indeed, it is now well established that the brain is a powerful regulator of skeletal homeostasis via a complex network of numerous players and pathways. In turn, bone via a bone-derived molecule, osteocalcin, appears as an important factor influencing the central nervous system by regulating brain development and several cognitive functions. In this paper we will discuss this complex and intimate relationship, as well as several pathologic conditions that may reinforce their potential interdependence.

Recombinant Uncarboxylated Osteocalcin Per Se Enhances Mouse Skeletal Muscle Glucose Uptake in both Extensor Digitorum Longus and Soleus Muscles

Emerging evidence suggests that undercarboxylated osteocalcin (ucOC) improves muscle glucose uptake in rodents. However, whether ucOC can directly increase glucose uptake in both glycolytic and oxidative muscles and the possible mechanisms of action still need further exploration. We tested the hypothesis that ucOC per se stimulates muscle glucose uptake via extracellular signal-regulated kinase (ERK), adenosine monophosphate-activated protein kinase (AMPK), and/or the mechanistic target of rapamycin complex 2 (mTORC2)-protein kinase B (AKT)-AKT substrate of 160 kDa (AS160) signaling cascade. Extensor digitorum longus (EDL) and soleus muscles from male C57BL/6 mice were isolated, divided into halves, and then incubated with ucOC with or without the pretreatment of ERK inhibitor U0126. ucOC increased muscle glucose uptake in both EDL and soleus. It also enhanced phosphorylation of ERK2 (Thr202/Tyr204) and AS160 (Thr642) in both muscle types and increased mTOR phosphorylation (Ser2481) in EDL only. ucOC had no significant effect on the phosphorylation of AMPKα (Thr172). The inhibition of ucOC-induced ERK phosphorylation had limited effect on ucOC-stimulated glucose uptake and AS160 phosphorylation in both muscle types, but appeared to inhibit the elevation in AKT phosphorylation only in EDL. Taken together, ucOC at the physiological range directly increased glucose uptake in both EDL and soleus muscles in mouse. The molecular mechanisms behind this ucOC effect on muscle glucose uptake seem to be muscle type-specific, involving enhanced phosphorylation of AS160 but limitedly modulated by ERK phosphorylation. Our study suggests that, since ucOC increases muscle glucose uptake without insulin, it could be considered as a potential agent to improve muscle glucose uptake in insulin resistant conditions.

Bone Turnover Markers in the Diagnosis and Monitoring of Metabolic Bone Disease

BACKGROUND

Disorders of bone metabolism, most notably osteoporosis, are highly prevalent and predispose to fractures, causing high patient morbidity and mortality. Diagnosis and monitoring of bone metabolic defects can present a major challenge as these disorders are largely asymptomatic and radiographic measures of bone mass respond slowly to changes in bone physiology.

CONTENT

Bone turnover markers (BTMs) are a series of protein or protein derivative biomarkers released during bone remodeling by osteoblasts or osteoclasts. BTMs can offer prognostic information on fracture risk that supplements radiographic measures of bone mass, but testing using BTMs has to take into account the large number of preanalytic factors and comorbid clinical conditions influencing BTM levels. BTMs respond rapidly to changes in bone physiology, therefore, they have utility in determining patient response to and compliance with therapies for osteoporosis.

SUMMARY

BTMs are a useful adjunct for the diagnosis and therapeutic monitoring of bone metabolic disorders, but their use has to be tempered by the known limitations in their clinical utility and preanalytic variables complicating interpretation.

Effect of gamma-carboxylase inhibition on serum osteocalcin may be partially protective against developing diabetic cardiomyopathy in type 2 diabetic rats

AIMS

To investigate the possible protective effect of elevated undercarboxylated osteocalcin on diabetic cardiomyopathy mechanisms and risk factors.

METHODS

In all, 32 male rats were divided into four groups: control, diabetic, diabetic warfarin and normal warfarin-treated groups. Isolated heart functions were assessed; fasting serum insulin, glucose and glycosylated haemoglobin, homeostasis model assessment insulin resistance and lipid profile were investigated. Serum undercarboxylated osteocalcin and adiponectin were also measured. In cardiac tissue, malondialdehyde content, acyl-CoA dehydrogenase gene expression, Bax/Bcl2 ratio, sarcoendoplasmic reticulum calcium ATPase and osteocalcin receptor (G protein-coupled receptor family C group 6 member A) genes expression were investigated.

RESULTS

Prophylactic elevation of undercarboxylated osteocalcin was accompanied by improved insulin sensitivity and lipid profile, increased serum adiponectin, upregulated myocardial osteocalcin receptor with preserved left ventricular function, decreased cardiac malondialdehyde content, acyl-CoA dehydrogenase and Bax/Bcl2 ratio.

CONCLUSION

Undercarboxylated osteocalcin was suggested to have protective effects against diabetic cardiomyopathy, possibly through direct action on upregulated G protein-coupled receptor family C group 6 member A and indirectly via adiponectin. These effects may be mediated through antagonizing oxidative stress and apoptosis.

Influence of carboxylation on osteocalcin detection by mass spectrometry

RATIONALE

Osteocalcin is a small, abundant bone protein that is difficult to detect using high-throughput tandem mass spectrometry (MS/MS) proteomic approaches from bone protein extracts, and is predominantly detected by non-MS immunological methods. Here, we analyze bovine osteocalcin and its post-translational modifications to determine why a protein of this size goes undetected.

METHODS

Osteocalcin was purified from cow bone using well-established methods. Intact osteocalcin or trypsin-digested osteocalcin were separated using an Agilent 1200 series high-performance liquid chromatography (HPLC) system and analyzed using a ThermoScientific LTQ-Orbitrap XL after fragmentation with higher-energy collision dissociation. Data were analyzed using Mascot or Prosight Lite.

RESULTS

Our results support previous findings that the cow osteocalcin has up to three carboxylations using both intact osteocalcin and digested forms. Using Mascot, we were able to detect osteocalcin peptides, but no fragments that localized the carboxylations. Full annotation using Prosight Lite of the intact (three carboxylations), N-terminal peptide (one carboxylation), and middle peptide (two carboxylations) showed complete fragmentation was present, but complete neutral loss was observed.

CONCLUSIONS

Osteocalcin carboxylation, and its associated neutral losses, makes high-throughput detection of this protein challenging; however, alternative fragmentation or limited purification can overcome these challenges. Copyright © 2016 John Wiley & Sons, Ltd.

Nerve Growth Factor, Brain-Derived Neurotrophic Factor and Osteocalcin Gene Relationship in Energy Regulation, Bone Homeostasis and Reproductive Organs Analyzed by mRNA Quantitative Evaluation and Linear Correlation Analysis

Nerve Growth Factor (NGF)/Brain-derived Neurotrophic Factor (BDNF) and osteocalcin share common effects regulating energy, bone mass, reproduction and neuronal functions. To investigate on the gene-relationship between NGF, BDNF, and Osteocalcin we compared by RT-PCR the transcript levels of Ngf, Bdnf and Osteocalcin as well as of their receptors p75NTR/NTRK1, NTRK2, and Gprc6a in brain, bone, white/brown adipose tissue (WAT/BAT) and reproductive organs of 3 months old female and male mice. Brain and bone were used as positive controls for NGF/BDNF and Osteocalcin respectively. The role of oxitocin(Oxt) and its receptor(Oxtr) was also investigated. Ngf expression shows an opposite trend compared to Bdnf. Ngf /p75NTR expression is 50% higher in BAT than brain, in both genders, but lower in bone. In contrast, Bdnf expression in bone is higher than in brain, but low in BAT/WAT. We found Osteocalcin gene expressed in brain in both genders, but Gprc6a expression is low in brain and BAT/WAT. As expected, Gprc6a gene is expressed in bone. Oxt gene was markedly expressed in brain, Oxtr in the ovaries and in fat and bone in both genders. Ngf is highly expressed in reproductive tissues and p75NTR mRNA levels are respectively 300, 100, and 50% higher in testis/ovaries/uterus than in brain. In contrast, BDNF genes are not expressed in reproductive tissues. As expected, Gprc6a is expressed in testis but not in the ovaries/uterus. A significant correlation was found between the expression levels of the gene ligands and their receptors in brain, BAT and testis suggesting a common pathway of different genes in these tissues in either male and female. Changes in the expression levels of osteocalcin, Ngf, or Bdnf genes may mutually affect the expression levels of the others. Moreover, it may be possible that different ligands may operate through different receptor subtypes. Oxt and Oxtr failed to show significant correlation. The up-regulation of Ngf /p75NTR in BAT is consistent with NGF as an energy regulator and with BDNF regulating bone.

Osteocalcin is necessary and sufficient to maintain muscle mass in older mice

Objective

A decrease in muscle protein turnover and therefore in muscle mass is a hallmark of aging. Because the circulating levels of the bone-derived hormone osteocalcin decline steeply during aging in mice, monkeys and humans we asked here whether this hormone might regulate muscle mass as mice age.

Methods

We examined muscle mass and strength in mice lacking osteocalcin (Ocn−/−) or its receptor in all cells (Gprc6a−/−) or specifically in myofibers (Gprc6a_Mck−/−) as well as in 9 month-old WT mice receiving exogenous osteocalcin for 28 days. We also examined protein synthesis in WT and Gprc6a−/− mouse myotubes treated with osteocalcin.

Results

We show that osteocalcin signaling in myofibers is necessary to maintain muscle mass in older mice in part because it promotes protein synthesis in myotubes without affecting protein breakdown. We further show that treatment with exogenous osteocalcin for 28 days is sufficient to increase muscle mass of 9-month-old WT mice.

Conclusion

This study uncovers that osteocalcin is necessary and sufficient to prevent age-related muscle loss in mice.

•

Osteocalcin is necessary to maintain muscle mass in older mice.

•

Osteocalcin is sufficient to increase muscle mass in older mice.

•

Osteocalcin promotes protein synthesis in myotubes.